Today's wireless cellular systems have been designed to handle very efficient data transfer between a single user, or rather a single User Equipment (UE), and a single Evolved universal terrestrial radio access network Node (eNB). These solutions are sufficient at today's carrier frequencies close to 1-2 GHz. In future fifth generation cellular system (5G) a shift towards higher carrier frequencies is a necessity, to be able to utilize the available spectrum, thereby achieving a higher capacity overall.
A consequence of the move to higher carrier frequencies is that the radio propagation is transformed from “diffuse” scattering to beamlike propagation. This leads to sharp diffraction effects and increasingly heavy radio shadowing behind obstacles. This makes it more difficult to obtain uniform coverage from a single eNB. The implication is a need to transmit from multiple, non-co-located, transmit points to cover a single cell. Such massive multi-point transmission is generally considered to become a cornerstone in future 5G radio access.
It should be noted that multi-point transmission is also considered for the present fourth generation cellular system (4G) Long-Term Evolution (LTE) system, however, the need and also the massiveness of the solutions for 4G are believed to be less than those of future 5G cellular systems. Also third generation cellular system (3G) multi-point transmission systems are feasible.
In a multi-point system, and in particular in a massive multi-point transmission system, all transmit points need to have access to timely data to transmit at every point in time and in many applications that data is closely related to data simultaneously being transmitted from other transmit points. This is e.g. the case for streaming video, in which case the data for a specific part of the video are needed at the same time in the receiving UE. It should be noted here that the different transmit points may transmit different data, or the same data for diversity gain, or a mix of these alternatives.
One mechanism for achieving multi-point transmission is illustrated in FIG. 1. That particular figure depicts an architecture of the 3G Wideband Code-Division Multiple Access (WCDMA) system. Similar architectures in 4G and 5G would involve nodes from both the Radio Access Network (RAN) and the Core Network (CN). FIG. 1 illustrates multipoint data flow, where splitting and recombination of a single flow via multiple flows at the physical layer is performed. A Radio Network Controller (RNC) 10 comprises a flow splitter 12, which splits a data flow 9 that is incoming to the RNC 10 into split data flows 13. The split data flows are provided to different transmission points 39, in this figure represented by a multitude of Radio Base Stations (RBS) 30:1, 30:2, . . . , 30:n. The split data flows are transmitted to a UE 20, in which the received split data flows 23 are recombined in a flow recombiner 22.
One key issue that may arise, particularly in heterogeneous multiflow systems, relates to synchronization of received data. Data, received by the flow splitter 12, will in the best case be an ordered set of packets that needs to be transmitted to the UE 20. However, due to non-uniform delays in the individual flows, the packets received by the UE 20 will in general be out of order. These delay variations may be the result of varying queuing delays in the RBS, varying transport network delays, e.g. due to congestion, and/or varying radio link quality, causing RBS buffer size variation.
Small timing errors between packets that are received are automatically handled by the Transmission Control Protocol (TCP) protocol which reorders the packets to resynchronize and create the correct data sequence. However, if the asynchronism is too large in comparison with the buffering capacity, TCP will register this as an error, and request retransmission of several packets. In some implementations of TCP, this may cause retransmission of out of sequence packets already received, as well as packets still in flight. This will then affect the user experience in a negative way, causing e.g. the streaming video to display erroneously.
There is therefore a need for an improvement of multi-point transmission, in terms of both capacity and user experience.